Switching amplifiers are common for many different applications, such as pulse generators. One application for a switching amplifier is in the audio field. Often a switching amplifier is used as an Analog-to-Digital Converter (ADC), or as a digital signal driver for an acoustic device, such as a speaker. One type of switching amplifier is commonly referred to as a class-D amplifier. Although various embodiments described herein may be discussed with relation to a class-D amplifier, one of ordinary skill will recognize that the embodiments may be equally useful with other switching amplifier configurations.
A class-D amplifier is an electronic amplifier where internal power devices, such as Metal-On-Silicon Field-Effect-Transistors (MOSFETs), are operated as binary switches. The MOSFETs are often driven to be either fully on or fully off. Ideally, zero time is spent transitioning between those two states. Class D amplifiers work by generating a variable duty cycle square wave of which the low-frequency portion of the spectrum nearly resembles the desired output signal, and of which the high-frequency portion serves no purpose other than to make the waveform binary so it can be amplified by switching the power devices.
The switching amplifier often includes a PMOS transistor and an NMOS transistor. The PMOS and the NMOS may switch on and off respectively to provide the switching output. Switching from one MOS to another needs to be done carefully so that direct current from PMOS to NMOS should be minimized, and the non-overlap time during which both PMOS and NMOS are off should also be minimized. It is also well known that electromagnetic interference (EMI) related to class D amplifier can be alleviated by slowing down the output slew rate.
To accurately control the PMOS and NMOS in a class D amplifier in a manner described above, it is desirable to use the amplifier output as feedback to control the slew rate. Specifically, it is desirable to drive the gate of the power transistors with a weak strength during the transition of output, but once output finishes its transition, it is desired to drive the gate of the power transistors with full force so that the overshoot, undershoot and energy loss all can be minimized. As used herein, a “weak” strength MOSFET may include a device that is small in physical dimensions relative to its complimentary device, or a device with lower charge-carrier mobility characteristics relative to its complimentary device. Prior art of using just a skewed inverter or conventional Schmitt trigger often cannot accurately control or robustly set the threshold voltage very close to the rail voltage, which is where the switching truly finishes.